The present invention relates generally to the field of radiation therapy devices such as medical linear accelerators equipped with portal imaging systems, or the like, typically used for providing Image Guided Radiation Therapy (IGRT), and more particularly to a system and method for providing automatic field size clipping to the allowed image area of the imager of the portal imaging system using the multi-leaf collimator of the linear accelerator.
IGRT uses patient positioning devices and radiation imaging technologies such as portal imaging to target and treat cancerous tumors more precisely. Prior to the use of IGRT, radiation oncologists contended with variations in patient positioning, including variations caused by a patient's respiratory motion. Inevitably, a margin of healthy tissue around a treatment site was treated with radiation. However, the use of IGRT allows a radiation oncologist to determine the exact positioning of a treatment site before the administration of radiation.
Portal imaging used in IGRT employs a radiation therapy device such as a medical linear accelerator equipped with portal imaging system. The portal imaging system employs an imager such as an amorphous-silicon based flat-panel imager, or the like, to obtain images that may be used for treatment verification and patient positioning. Such flat-panel imagers include an active image area which is irradiated by the linear accelerator to generate the portal image. This active imaging area is typically surrounded by radiation sensitive electronics, which if exposed to radiation may be damaged or degraded. As a result, it is desirable that these electronics be protected from exposure to radiation.
To protect the sensitive electronics of the flat-panel imager, portal imaging systems typically prevent the user from acquiring an image if the projection of the planned portal image field onto the flat-panel imager is larger than an allowed image area of the imager which is equal to or less than the active image area, or if the projection of the portal image field is not centered and would extend beyond this allowed image area. This limitation greatly reduces the usability of the portal imaging system.
Consequently, it would be desirable to allow users of medical linear accelerators equipped with portal imaging systems to acquire portal images by automatically clipping the planned field size in instances where the projection of the portal image field onto the flat-panel imager is larger than an allowed image area of the imager, or where the projection of the image field is not centered and would extend beyond this allowed image area.